Volunteer computing

Volunteer computing is type of distribute computing in which normal users volunteer to contribute their computing resources which are not in use by them for projects which require it. This is similar to grid computing but experts always asks to distinguish between grid and volunteer computing. In volunteer computing the users may be anonymous which is not generally possible for a grid contributer. But desktop grid computing do have close similarity to volunteer computing. Both are inseparably related to normal PC users across the globe.
Also, a non desktop grid generally refers to sharing of resources within and between organizations in which each of them is 'responsible' for the maintainance and smooth working of the grid. Also each organisation involved can act as the producer or consumer.

Volunteer computing on Berkeley wiki
Volunteer computing in Wikipedia